We will discuss the invention with reference to an example cellular system for which the invention is beneficial, namely IS-54-B Cellular System Dual-Mode Mobile Station--Base Station Compatibility Standard (800 MHz AMPS/TDMA cellular systems). Existing Multi-Channel Power Amplifiers (MCLPAs) used in IS-54-B cellular systems must function with both analog and digital modulation schemes. MCLPAs must provide a highly linear response over a large dynamic range. This wide dynamic range is required since the characteristics of the signals amplified by the MCLPA include a very large peak-to-average signal power.
Individual signals being fed into the amplifier have an inherently high peak-to-average signal power. The largest component of the peak signal power occurs whenever a modulation symbol goes through transition. Multiple input signals increase the total peak-to-average signal power whenever modulation transitions on multiple signals occur at the same time effectively increasing the amplitude of the peak transient.
AMPS analog transmission is based upon one traffic channel per Radio Frequency (RF) carrier. In TDMA mode, three traffic channels are imposed on one RF carrier using digital modulation techniques. The ensuing linearity requirements in TDMA mode call for a highly linear power amplifier to maintain modulation mask integrity.
A problem with TDMA modulation when applied to an MCLPA environment is periodic correlation of symbol transitions. In a typical system, a base station is connected to the rest of the wireless network by one or more T1 connections. Multiple signals are carried by each T1 link, and there is a correlation between the multiple signal streams in the form of synchronized data bit transitions. For example, bits arriving on a T1 span will have an inherent correlation to the T1 span clock when they arrive at the base station span adapter. Modulation symbols represent one or more data bits (in the case of IS-54-B one modulation symbol represents two data bits). Timing correlations between multiple data bit streams at the span adapter will translate into timing correlations of the subsequent modulation symbols.
This correlation sometimes increases the peak input power far above the average power, and drives the MCLPA into a less linear state, generating Intermodulation Distortion (IMD) above the levels specified in IS-54-B.
MCLPAs are typically advertised with a continuous maximum rated power using a Continuous Wave (CW) carrier, such as Maximum 50 Watts CW. In order to avoid this problem in the past, many approaches have been attempted to ensure that adequate linearity is maintained. For example, cellular operators running these MCLPAs in TDMA mode have had to reduce the input drive (effectively the overall output power) to ensure that adequate linearity is maintained at the peak signal power levels. However, reducing total output power negatively affects cost, performance, and network capacity.
The prior art has seen manufacturers of cellular base station amplifier systems attempt many solutions to this problem. These prior art solutions have focused on design concepts to increase amplifier linearity. Error amplification, feedback, pre-distortion and feed-forward control techniques are commonly employed to increase the range of linearity of the amplifiers in order to avoid these problems. All of these techniques to enhance linearity of the MCLPA are accompanied by increased cost. While necessary for TDMA systems, this increased cost offers little benefit during AMPS operation, as the required degree of linearity for analog transmission is much less than digital requirements needed to support TDMA signals.
Therefore, there exists a need for an improved system which will overcome these problems at a reduced cost.